WO1997003309A1

WO1997003309A1 - Ball-nut

Info

Publication number: WO1997003309A1
Application number: PCT/SE1996/000864
Authority: WO
Inventors: Bo Granbom
Original assignee: Bo Granbom
Priority date: 1995-07-11
Filing date: 1996-06-28
Publication date: 1997-01-30
Also published as: SE9502562D0; SE503473C2; AU6373496A; SE9502562L

Abstract

A ball-nut comprising a screw (1) with screw formed threads (2) and in a nut retained balls (11) cooperating with these, the balls running between two ball races (8) suspended perpendicularly to the shaft of the screw (1) and rotatably in the nut. The ball-nut is characterized in that each ball (11) has a radius being so much minor than the radius of the thread profile of the screw (1) and the ball treads (6) of the ball races (8), respectively, that the contact point (15) of the ball (11) with the thread (2) at a load on the ball-nut is situated on a tangent to it declined with an angle α being less than 90 degrees and not zero to the shaft (12) of the screw and the contact point (16) of the ball tread (6) of the corresponding ball race (8) situated on a tangent to it declined with an angle β, which is not zero, but minor or equal to the angle α towards the axis (12) of the screw.

Description

Ball-nut

The present invention relates to a new type of ball-nuts.

Ball-nuts, are used in those cases, where a high precision is desirable. The ball-nut provides further advantages such as a low initial resistance and a minimal need for greasing. The ball- nut, however, has several drawbacks, ofwhich the largest drawback in most applications is a limited speed, i.e. the rotational speed ofthe screw. Moreover, there is a risk for the nut to fasten at the end position. The Swedish patent 8702982-3 describes a ball-nut which can operate at a substantially higher speed and the ball-nut is allowed to rotate at up to 1.000 rpm. In designing the nut the power, with which the nut risks to fasten in an end position, might be reduced considerably in comparison with the type of a ball-nut known in the art. Even if this ball-nut then operates satisfactorily it has been shown that it operates with too large friction losses. This means that the efficiency ofthe ball-nut is limited to about 85 percent, i.e. the efficiency is not better than for the best ball-nut known today, the risk for an overheating at the same time being substantial with a thereby resulting blueing ofthe balls.

The present invention, presented in the characterizing parts ofthe claims, provides a ball-nut provided with the advantages ofthe above mentioned ball-nut , the ball-nut ofthe present invention at the same time operating with a low friction and with an efficiency of about 96 percent. Any risk for an overheating ofthe ball-nut is eliminated.

The invention will be described more in detail here below with examples in connection with the drawing, where Figure 1 schematically shows a section of a nut half with a screw half, and Figure 2 schematically shows a part ofthe ball-nut according to the invention in a larger scale.

Thus, the ball-nut consists of a screw 1 with threads 2, possibly provided with one or several entrances. The threads are only schematically shown in the figures. Moreover, the ball-nut consists of a nut 3, coaxially circumsizing the screw 1. The nut 3 comprises a sleeve 4 and two gable parts 5 each provided with an opening for the screw 1 and a ball tread 6 on the inward side, respectively. Ball races 8 are provided rotatively suspended in the nut by means of bearing balls 7, running in the ball treads 6. Each ball race 8 comprises a ball tread 9 cooperating with the bearing balls 7, as well as a ball tread 10. Between the screw 1 and the ball races 8, running in the screw threads, and the ball treads 10 ofthe ball races 8 balls are provided, obviously situated in a plane perpendicular to the shaft 12 of the screw 1 and rolling in each its thread, provided in said plane. Depending on the pitch ofthe thread and the number of thread entrances the number of balls 11 is varying.

With 13 are designated ball retainers, freely rotatable in the nut 3 together with the bearing balls 7, whereas 14 designates a ball retainer for the balls 11 firmly connected to the nut sleeve 4.

In operating a ball-nut the nut will in rotating the screw 1 by the engagement ofthe balls in the threads been axially displaced along the screw, being understood that the nut itself will not rotate. In a corresponding way the nut might of course be displaced and thus rotate the screw. Theoratically any rotation ofthe screw will roll the balls symmetrically in the threads and the balls races will at the same time uniformly rotate in an opposite direction to the screw. However, the problem is that these theoretical, ideal circumstances do not prevail. Indepen¬ dent ofthe demand for utmost close tolerances the ball-nuts are operating under heavy loads and with high speeds, resulting in heat dilatation and heat stresses. A large risk exists that the ball will roll against both sides ofthe thread groove resulting in a high friction and at the same time the ball races are affected by oppositely directed forces from the cooperating balls, e.g. as the ball-nut designed according to said SE 8702982-3. The balls will roll without any definiton contributing to an uneven wear ofthe treads and the balls.

The present invention has the obejct to solve the problems for this type of ball-nuts with a large friction resulting in a heavy heat release and the undefined movements ofthe balls in the ball treads.

This problem is solved in such a way that the ball-nut is provided with only one line of balls, the balls 11, running in the nut threads 2 cooperating with two ball races 8 freely rotatable in the nut and that the thread profile and the profile ofthe ball treads 10 ofthe corresponding ball races are provided with dimensions in relation to the balls 11 , that these during the operation ofthe ball-nut comprise three contact or rolling points, on one hand a point 15 on the side ofthe screw thread and on the other hand a point 16 on each ball tread 10 of the ball races 8. To guarantee this condition a certain gap has to exist between the ball and the thread sides, i.e. the thread groove should be somewhat larger than is required for the ball. This gap can be about 0.02 mm. In loading the ball-nut the ball rolling point in the thread thus will be displaced radially outwards, as seen from the shaft 12 ofthe screw 1, i.e. the rolling point is placed longer outwards on the thread side. Hereby the balls are forced radially outwards and thereby a good contact is provided with the ball races 8, see figure 2, in which the nut 3 possibly is actuated by an axial force from the right, being understood that the illustrated gaps are largely schematic. Thus, a well defined contact between the ball, the thread side and the ball races is established in three rolling points. However, for an absolute guarantee during all circumstances that these three rolling points or rolling lines are maintained at all times, the contact or rolling point ofthe ball 11 on the corrsponding ball race 8 should be situated at a point, where the ball radius declines with an angle β towards the shaft 12 ofthe screw. The angle β should then be equal or less than the angle a, being the declination ofthe contact or rolling point towards the shaft 12 of the screw. Thus, the force separating the ball races 8 from the balls 11 will be larger than the force acting between the balls 11 and the thread side in the rolling point 15 (for β = a the centrifugal force on the balls acts as a separating force). The balls 11 will operate in the steadily provided plane, defined by the three contact or rolling points 15 and 16, i.e. the balls 11 are rolling in a common plane without any friction losses in the screw 1 and in the ball races 8. The ball races 8 freely rotating in the nut, the races 8 thus will turn in opposite directions, when the balls are rolling in the screw threads on its thread side along the rolling line generated by the rolling points 15.

With 17 is designated locking and fastening pins for the fixation ofthe gable parts 5 onto the sleeve 4. Of course, this fixation might be achieved in many ways obviously known in the art.

To illustrate the unique low friction ofthe ball nut ofthe invention the efficiency for a ball nut is calculated as an example, the angle α being 45 degrees and the angle β 30 degrees. The ball-nut has a diameter of 25 mm, a pitch of of 36 mm and eight thread entrances (i.e. eight balls). The gap between the balls and the both screw thread sides is about 0.02 mm.

The only friction losses Q to be considered here are coming from the ball retainer 14 and is calculated according to Q = C x μ x L

where L is the length ofthe distance the ball will roll when the screw rotates one turn and C is the pressure ofthe screw against the ball retainer for a certain load and μ is the coefficient of friction between the ball and the ball retainer.

The efficiency θ is calculated according to θ = 100 - O ^• 100 . E

Assuming the load in the example being 100 kg the ball is pressed against the screw side at an angle of 45 degrees and the force radially acting on the screw will thus become a = 50 kg. The screw pitch angle γ will then be

Tgγ -= 36 where γ = 26 degrees

23,7xτr

L = 36 + m.lxir = 0,0827 m and 1000

C = a ^• sin γ ^• cos = 50 ^• 0,438 ^• 0,899 = 19,69 kp, and

distributed on 8 balls is 2.46 kp per ball.

μ is chosen 0.1 and for one turn ofthe screw E will be 3.6 kpm. By this is obtained

9 = 100 - 19.69 ^• 0.1 ^• 0.0827 ^• 100 = 95,48% 3,6

The angles α and β (figure 2) might of course possibly have other values than said above, but β should always be less than a.

Claims

1. A ball-nut comprising a screw (1) with screw formed threads (2) and in a nut (3) retained balls (11) cooperating with these, the balls running between two ball races (8) suspended perpendicularly to the shaft (12) ofthe screw (1) and rotatably in the nut (3), characterized in that each ball (11) has a radius being so much less than the radius ofthe thread profile ofthe screw (1) and the ball treads (6) ofthe ball races (8), respectively, that the contact point (15) ofthe ball (11) with the thread (2) at a load on the ball-nut is situated on a tangent to it declined with an angle α being less than 90 degrees and not zero to the shaft (12) ofthe screw and the contact point (16) ofthe ball tread (6) ofthe corresponding ball race (8) situated on a tangent to it declined with an angle β, which is not zero, but minor or equal to the angle α towards the axis (12) ofthe screw.

2. A ball-nut according to claim 1 , characterized in that the ball races (8) are suspended in the nut (3) by means of bearing balls (7).

3. A ball-nut according to claim 2, characterized in that a ball retainer (4) is arranged between the balls (11) and is steadily fixed to the nut (3).

4. A ball-nut according to anyone ofthe preceding claims, characterized in that the angle a is about 45 degrees.

5. A ball-nut according to anyone ofthe preceding claims, characterized in that the angle β is about 30 degrees.